Efficient scavenging of Criegee intermediates on water by surface-active cis-pinonic acid.

cis-Pinonic acid (CPA), the main product of the atmospheric oxidation of biogenic α-pinene emissions and a major component of secondary organic aerosol (SOA), is a potentially key species en route to extremely low volatility compounds. Here, we report that CPA is an exceptionally efficient scavenger of Criegee intermediates (CIs) on aqueous surfaces. Against expectations, millimolar CPA (a surface-active C10 keto-carboxylic acid possessing a rigid skeleton) is able to compete with 23 M bulk water for the CIs produced in the ozonolysis of sesquiterpene solutes by O3(g) on the surface of a water:acetonitrile solvent. The significance of this finding is that CPA reactions with sesquiterpene CIs on the surface of aqueous organic aerosols would directly generate C25 species. The finding that competitive reactions at the air-liquid interface depend on interfacial rather than bulk reactant concentrations should be incorporated in current chemical models dealing with SOA formation, growth and aging.

Low intensity, continuous wave photodoping of ZnO quantum dots - photon energy and particle size effects.

The unique properties of semiconductor quantum dots (QDs) have found application in the conversion of solar to chemical energy. How the relative rates of the redox processes that control QD photon efficiencies depend on the particle radius (r) and photon energy (Eλ), however, is not fully understood. Here, we address these issues and report the quantum yields (Φs) of interfacial charge transfer and electron doping in ZnO QDs capped with ethylene glycol (EG) as a function of r and Eλ in the presence and absence of methyl viologen (MV2+) as an electron acceptor, respectively. We found that Φs for the oxidation of EG are independent of Eλ and photon fluence (φλ), but markedly increase with r. The independence of Φs on φλ ensures that QDs are never populated by more than one electron-hole pair, thereby excluding Auger-type terminations. We show that these findings are consistent with the operation of an interfacial redox process that involves thermalized carriers in the Marcus inverted region. In the absence of MV2+, QDs accumulate electrons up to limiting volumetric densities ρe,∞ that depend sigmoidally on excess photon energy E* = Eλ - EBG(r), where EBG(r) is the r-dependent bandgap energy. The maximum electron densities: ρev,∞ ∼ 4 × 1020 cm-3, are reached at E* > 0.5 eV, independent of the particle radius.

Reactions of Criegee Intermediates with Alcohols at Air-Aqueous Interfaces.

The fate of Criegee intermediates (CIs) from the gas-phase ozonolysis of unsaturated organic compounds in the troposphere is largely controlled by their reactions with water vapor. We recently found that against all expectations carboxylic acids compete at millimolar concentrations with water for CIs at the air-liquid interface of aqueous organic media. This outcome is consistent with both the low water concentration in the outermost interfacial layers and the enrichment of the competing acids therein. Here we show, via online electrospray mass spectrometric detection, that CIs generated in situ in the fast ozonolysis of sesquiterpenes (C15H24) on the surface of water:acetonitrile microjets react with n ≥ 4 linear alcohols CnH2n+1OH to produce high molecular weight C15+n ethers in one step. The OH group of 1-octanol proved to be ∼25 times less reactive than that of n-octanoic toward CIs at the same bulk molar concentration, revealing that the reactivity of hydroxylic species depends on both acidities and interfacial affinities. CI interfacial reactions with surface-active hydroxylic species, by bypassing water, represent shortcuts to molecular complexity in atmospheric aerosols.

Halogen Radical Chemistry at Aqueous Interfaces.

Halogens play key roles in the chemical composition of marine boundary layers, the free troposphere and the stratosphere. Atmospheric halogen chemistry is dominated by reactions between gas-phase and aqueous species on the surfaces of the ocean and marine aerosol. The mechanisms of interfacial halogen radical/halide reactions, however, are not fully understood, partly due to the dearth of techniques for in situ monitoring of the products and intermediates of fast interfacial halogen radical reactions. Here, we report the online electrospray mass spectrometric identification of the species produced on the surface of aqueous Br(-) and I(-) microjets collided by I(•)(g) pulses generated from the 266 nm laser photolysis of CH3I/O2/N2 gas mixtures. Mass-specific identification of intermediates and products in D2O and H2(18)O solutions and their dependences on I(•)(g) fluxes let us outline mechanisms of formation. We found that the uptake of I(•)(g) on the surface of Br(-) and I(-) microjets (effective uptake coefficient γeff ≥ 2 × 10(-4)) yields IBr(•)(-)/I2(•)(-) radical intermediates, which rapidly react with additional I(•) to produce trihalides I2Br(-)/IBr2(-)/I3(-) plus I3On(-) (n = 1, 2) species within ∼10 µs. Our findings point to a new halogen activation pathway initiated by photogenerated I(•).

A phase II study on the efficacy and safety of procedural analgesia with fentanyl buccal tablet in cancer patients for the placement of indwelling central venous access systems.

BACKGROUND: Pain in cancer patients is often related to oncologic therapies and diagnostic procedures. The placement of fully implantable venous access systems is a very common procedure in oncology patients. Local anaesthesia is the method most commonly used to overcome pain related to this surgical procedure, but the local anaesthetic may be unable to completely eradicate all pain. This study investigates the effectiveness and safety of fentanyl buccal tablet (FBT), administered by OraVescent® technology, in reducing procedural pain related to the placement of indwelling central venous access systems (Ports) in opioid-naïve cancer patients. METHODS: Inpatients who required an indwelling vascular access (Port) were preoperatively assessed with a self-assessment questionnaire on anxiety and pain. A 100 µg FBT was administered 10 min before preparation of the operating field. A self-assessment scale for pain experienced during the procedure was administered at the end of the procedure. Vital signs and the presence of any side effects or bothersome symptoms were monitored during the procedure, at the end, and 4 h later. RESULTS: From October 2012 to June 2014, 65 patients were enrolled in the study. A total of 61 (93.9 %) patients perceived no or a little pain during the procedure. Four patients (6.2 %) reported a lot of pain. No patient reported very severe pain. This data is significant in terms of the lower than expected presence of pain (Fisher test p = 0.0018) as assessed in our previous experience without procedural analgesia. The most common side effects of FBT was drowsiness, experienced by 28 patients at the end of the procedure (43.1 %), significantly reduced (p < 0.01) to 8 patients after 4 h (12.5 %). Nausea was present in 6 cases at the end of the procedure (9.2 %) and in 7 cases 4 h later (10.9 %). Vomiting was present in 3 cases at the end (4.7 %) and in 2 other patients after 4 h (7.8 %). No significant change of vital parameters was observed between the baseline and the subsequent measurements in all patients studied. CONCLUSIONS: The significant improvement in the number of patients experiencing little or no pain, accompanied by a lower number of non-severe side effects, suggests that FBT is a valid, practical and safe method of procedural analgesia. It will be necessary to perform further studies, taking into account the need for standard antiemetic pre-medication to minimise the incidence of nausea and vomiting.

Homogeneous reduction of CO2 by photogenerated pyridinyl radicals.

We report that 1-hydropyridinyl radicals (1-PyH(•)) photogenerated in solution react with dissolved CO2 en route to its 2e(-) reduction into carboxylic acids. The 254 nm excitation of pyridine (Py) in deaerated 2-PrOH/H2O mixtures saturated with 1 atm of CO2 yields a suite of products, among which we identified Na(HCOO)2(-) (m/z(-) = 113), C5H6NCOO(-) (m/z(-) = 124), and C5H10O2NCOO(-) (m/z(-) = 160) species by electrospray ionization mass spectrometry. These products demonstrably contain carboxylate functionalities that split CO2 neutrals via collisionally induced dissociation. We infer that 1-PyH(•) [from (1) (3)Py* + 2-PrOH → 1-PyH(•) + (•)PrOH] adds to CO2, in competition with radical-radical reactions, leading to intermediates that are in turn reduced by (•)PrOH into the observed species. The formation of carboxylates in this system, which is shown to require CO2, Py, 2-PrOH, and actinic radiation, amounts to the homogeneous 2e(-) reduction of CO2 by 2-PrOH initiated by Py*. We evaluate a rate constant (2) k2(1-PyH(•) + CO2 → (•)Py-1-COOH) ≈ O (10) M(-1) s(-1) and an activation energy E2 ≥ 9 kcal mol(-1) that are compatible with thermochemical estimates for this reaction.

OH radical-initiated chemistry of isoprene in aqueous media. Atmospheric implications.

The fate of isoprene (2-methyl-1,3-butadiene, ISO) emissions into the atmosphere is not fully understood. Increasing awareness that ISO is only partially processed in the gas-phase has turned attention to its reactive uptake by fog, cloud, and aerosol droplets. A hydrophobic gas, ISO would preferentially partition to the surface rather than the bulk of aqueous media. Such media normally contain dissolved O2 and water-soluble unsaturated organics and support •OH generation rates (from the solar photolysis of dissolved H2O2) that are several orders of magnitude larger than in the gas-phase. Thus, ISO should be converted therein to heavier products rather than into the C4-C5 volatile compounds produced in the gas-phase. Here we substantiate such a scenario by reporting that the λ > 305 nm photolysis of H2O2 in dilute aqueous ISO solutions yields C10H15OH species as primary products, whose formation both requires and is inhibited by O2. A minimum of seven C10H15OH isomers are resolved by reverse-phase high-performance liquid chromatography and detected as MH(+) (m/z = 153) and MH(+)-18 (m/z = 135) signals by electrospray ionization mass spectrometry. Our findings are consistent with the addition of •OH to ISO, followed by HO-ISO• reactions with ISO in competition with O2, leading to second generation HO(ISO)2• radicals that terminate as C10H15OH via ß-H abstraction by O2. We show that a significant fraction of gas-phase olefins should be converted into less volatile species via this process on wet airborne particles.

Acidity enhances the formation of a persistent ozonide at aqueous ascorbate/ozone gas interfaces.

The pulmonary epithelium, like most aerial biosurfaces, is naturally protected against atmospheric ozone (O(3)) by fluid films that contain ascorbic acid (AH(2)) and related scavengers. This mechanism of protection will fail, however, if specific copollutants redirect AH(2) and O(3)(g) to produce species that can transduce oxidative damage to underlying tissues. Here, the possibility that the synergistic adverse health effects of atmospheric O(3)(g) and acidic particulate matter revealed by epidemiological studies could be mediated by hitherto unidentified species is investigated by electrospray mass spectrometry of aqueous AH(2) droplets exposed to O(3)(g). The products of AH(2) ozonolysis at the relevant air-water interface shift from the innocuous dehydroascorbic acid at biological pH to a C(4)-hydroxy acid plus a previously unreported ascorbate ozonide (m/z = 223) below pH approximately 5. The structure of this ozonide is confirmed by tandem mass spectrometry and its mechanism of formation delineated by kinetic studies. Present results imply enhanced production of a persistent ozonide in airway-lining fluids acidified by preexisting pathologies or inhaled particulate matter. Ozonides are known to generate cytotoxic free radicals in vivo and can, therefore, transduce oxidative damage.

Rhomboid domain containing 2 (RHBDD2): a novel cancer-related gene over-expressed in breast cancer.

In the course of breast cancer global gene expression studies, we identified an uncharacterized gene known as RHBDD2 (Rhomboid domain containing 2) to be markedly over-expressed in primary tumors from patients with recurrent disease. In this study, we identified RHBDD2 mRNA and protein expression significantly elevated in breast carcinomas compared with normal breast samples as analyzed by SAGE (n=46) and immunohistochemistry (n=213). Interestingly, specimens displaying RHBDD2 over-expression were predominantly advanced stage III breast carcinomas (p=0.001). Western-blot, RT-PCR and cDNA sequencing analyses allowed us to identify two RHBDD2 alternatively spliced mRNA isoforms expressed in breast cancer cell lines. We further investigated the occurrence and frequency of gene amplification and over-expression affecting RHBDD2 in 131 breast samples. RHBDD2 gene amplification was detected in 21% of 98 invasive breast carcinomas analyzed. However, no RHBDD2 amplification was detected in normal breast tissues (n=17) or breast benign lesions (n=16) (p=0.014). Interestingly, siRNA-mediated silencing of RHBDD2 expression results in a decrease of MCF7 breast cancer cells proliferation compared with the corresponding controls (p=0.001). In addition, analysis of publicly available gene expression data showed a strong association between high RHBDD2 expression and decreased overall survival (p=0.0023), relapse-free survival (p=0.0013), and metastasis-free interval (p=0.006) in patients with primary ER-negative breast carcinomas. In conclusion, our findings suggest that RHBDD2 over-expression behaves as an indicator of poor prognosis and may play a role facilitating breast cancer progression.

Molecular control of reactive gas uptake "on water".

Reactive gas uptake on environmentally realistic aqueous surfaces is expected to be affected by a combination of multiple interactions. This issue is herein explored in experiments where the formation of Me(3)NH(+) on neat and doped water microjets exposed to Me(3)N(g) is monitored within <1 ms by online electrospray ionization mass spectrometry as a function of pH of the bulk liquid (pH(BLK)). Notably, Me(3)N(g) is protonated on the surface of neat water microjets below pH(BLK) approximately 4, rather than at pH(BLK) less than or approximately pK(A)(Me(3)NH(+)) = 9.8 as in bulk water. Me(3)N(g) uptake is significantly enhanced by anionic surfactants and fulvic acid (a surrogate of complex natural organic matter) above pH(BLK) approximately 4, uniformly depressed by cationics (which otherwise counteract FA effects), and unaffected by n-octanol. The direct hydrogen isotope effects associated with enhanced uptake of Me(3)N(g) on H(2)O/D(2)O microjets implicate a process controlled by proton transfer from interfacial donors whose coverage is electrostatically modulated by ionic headgroups. The finding that the combined effect of fulvic acid and tetrabutylammonium bromide closely matches the geometric mean of their separate effects on TMA uptake is evidence of strong dopant interactions.

Ozone oxidizes glutathione to a sulfonic acid.

Biosurfaces are universally covered with fluid microfilms containing reduced glutathione (GSH) and other antioxidants whose putative roles include the detoxification of ambient ozone (O(3)). It is generally believed that O(3) accepts an electron from the thiolate GS(2-) function [pK(a)(GS(-)) = 8.8] of GSH to produce thiyl GS(*-) radicals en route to the disulfide GSSG. Here, we report novel electrospray mass spectrometry experiments showing that sulfonates (GSO(3)(-)/GSO(3)(2-)), not GSSG, are the exclusive final products on the surface of aqueous GSH microdroplets exposed to dilute O(3)(g) for approximately 1 ms. The higher reactivity of the thiolate GS(2-) toward O(3)(g) over the thiol GS(-) is kinetically resolved in this time frame due to slow GS(-) acid dissociation. However, our experiments also show that O(3) will be largely scavenged by the more reactive ascorbate coantioxidant in typical interfacial biofilms. The presence of GSSG and the absence of GSO(3)(-)/GSO(3)(2-) in extracellular lining fluids are therefore evidence of GSH oxidation by species other than O(3).

Absorption of inhaled NO(2).

Nitrogen dioxide (NO(2)), a sparingly water-soluble pi-radical gas, is a criteria air pollutant that induces adverse health effects. How is inhaled NO(2)(g) incorporated into the fluid microfilms lining respiratory airways remains an open issue because its exceedingly small uptake coefficient (gamma approximately 10(-7)-10(-8)) limits physical dissolution on neat water. Here, we investigate whether the biological antioxidants present in these fluids enhance NO(2)(g) dissolution by monitoring the surface of aqueous ascorbate, urate, and glutathione microdroplets exposed to NO(2)(g) for approximately 1 ms via online thermospray ionization mass spectrometry. We found that antioxidants catalyze the hydrolytic disproportionation of NO(2)(g), 2NO(2)(g) + H(2)O(l) = NO(3)(-)(aq) + H(+)(aq) + HONO, but are not consumed in the process. Because this function will be largely performed by chloride, the major anion in airway lining fluids, we infer that inhaled NO(2)(g) delivers H(+), HONO, and NO(3)(-) as primary transducers of toxic action without antioxidant participation.

Simultaneous detection of cysteine sulfenate, sulfinate, and sulfonate during cysteine interfacial ozonolysis.

Sulfenic acids (RSOH) are reactive intermediates in the oxidation of protein cysteines. Among cysteine oxoforms, RSOH represent redox-reversible species that can thus participate in regulation and signaling mechanisms and play key roles in enzyme catalysis and antioxidant activity. How the cysteine (CyS) thiol groups of the human surfactant protein that lines the lung epithelium react with inhaled ozone is deemed critical in preserving structural integrity and immune functions. Here we report the simultaneous detection, by online thermospray ionization mass spectrometry, of cysteine sulfenate (CySO(-)) and the overoxidized cysteine sulfinate (CySO(2)(-)) and cysteine sulfonate (CySO(3)(-)) species on the surface of aqueous CyS microdroplets exposed to O(3)(g) for <1 ms. These species are produced by rapid, sequential O-atom additions whose relative rates are herein quantified for the first time. From the pH-dependence of ozonation rates, we derive pK(a)(CySOH) = 7.6 +/- 0.3 < pK(a)(CyS) = 8.3.

Enrichment factors of perfluoroalkyl oxoanions at the air/water interface.

The refractory, water-bound perfluoro-n-alkyl carboxylate F(CF2)nCO2- and sulfonate F(CF2)nSO3- surfactant anions reach remote locations by mechanisms that are not well understood. Here we report experiments in which the relative concentrations of these anions on the surface of microdroplets produced by nebulizing their aqueous solutions are measured via electrospray ionization mass spectrometry. Enrichment factors f (relative to Br-: f(Br-) identical with 1) increase with n, asymptotically reaching f[F(CF2)nSO3-] approximately 2 f[F(CF2)nCO2-] approximately 200 f(Br-) values above n approximately 8. The larger f values for F(CF2)nSO3- over their F(CF2)nCO2- congeners are consistent with a closer approach of the bulkier, less hydrated -SO3- headgroup to the air/water interface. A hyperbolic, rather than the predicted linear log f[F(CF2)nCO2-] vs n dependence suggests the onset of conformational restrictions to interfacial enrichment above n approximately 4. Marine aerosols produced from contaminated ocean surface waters are therefore expected to be highly enriched in F(CF2)nCO2-/F(CF2)nSO3- species.

How phenol and alpha-tocopherol react with ambient ozone at gas/liquid interfaces.

The exceptional ability of alpha-tocopherol (alpha-TOH) for scavenging free radicals is believed to also underlie its protective functions in respiratory epithelia. Phenols, however, can scavenge other reactive species. Herein, we report that alpha-TOH/alpha-TO(-) reacts with closed-shell O(3)(g) on the surface of inert solvent microdroplets in < 1 ms to produce persistent alpha-TO-O(n)(-)(n = 1-4) adducts detectable by online thermospray ionization mass spectrometry. The prototype phenolate PhO(-), in contrast, undergoes electron transfer under identical conditions. These reactions are deemed to occur at the gas/liquid interface because their rates: (1) depend on pH, (2) are several orders of magnitude faster than within microdroplets saturated with O(3)(g). They also fail to incorporate solvent into the products: the same alpha-TO-O(n)(-) species are formed on acetonitrile or nucleophilic methanol microdroplets. alpha-TO-O(n = 1-3)(-) signals initially evolve with [O(3)(g)] as expected from first-generation species, but alpha-TO-O(-) reacts further with O(3)(g) and undergoes collisionally induced dissociation into a C(19)H(40) fragment (vs C(19)H(38) from alpha-TO(-)) carrying the phytyl side chain, whereas the higher alpha-TO-O(n > or = 2)(-) homologues are unreactive toward O(3)(g) and split CO(2) instead. On this basis, alpha-TO-O(-) is assigned to a chroman-6-ol (4a, 8a)-ene oxide, alpha-TO-O(2)(-) to an endoperoxide, and alpha-TO-O(3)(-) to a secondary ozonide. The atmospheric degradation of the substituted phenols detected in combustion emissions is therefore expected to produce related oxidants on the aerosol particles present in the air we breathe.

Acid dissociation versus molecular association of perfluoroalkyl oxoacids: environmental implications.

Perfluorooctanoate (PFO) and perfluorooctanesulfonate (PFOS) surfactant anions, once released, may rapidly reach remote regions. This phenomenon is puzzling because the water-bound anions of strong F-alkyl acids should be largely transported by slow oceanic currents. Herein, we investigate whether these hydrophobic F-alkyl oxoanions would behave anomalously under environmental conditions, as suggested elsewhere. Negative electrospray ionization mass spectra of micromolar aqueous PFO or PFOS solutions from pH 1.0 to 6.0 show (1) m/z = 499 (PFOS) signals that are independent of pH and (2) m/z = 413 (PFO) and 369 (PFO-CO(2)) signals, plus m/z = 213 (C(3)F(7)CO(2)(-)) and 169 (C(3)F(7)(-)) signals at higher collision energies, and, below pH approximately 4, m/z = 827 signals from a remarkably stable (PFO)(2)H(-) cluster that increase with decreasing pH. Since the sum of the m/z = 369, 413, and 827 signal intensities is independent of pH, that is, effectively encompasses all major species, we infer that pK(a)(PFOSA) < 1.0 and pK(a)(PFOA) < 1.0. We also derive K(2) < or = 4 x 10(7) M(-2) for the clustering equilibrium 2PFO + H(+) <==> (PFO)(2)H. Thus, although (PFO)(2)H is held together by an exceptionally strong homonuclear covalent hydrogen bond, neither PFOS nor PFO will associate or protonate significantly at environmentally relevant subnanomolar concentrations above pH approximately 1.

Anion-catalyzed dissolution of NO2 on aqueous microdroplets.

Fifty-seven years after NO(x) (NO + NO(2)) were identified as essential components of photochemical smog, atmospheric chemical models fail to correctly predict *OH/HO(2)* concentrations under NO(x)-rich conditions. This deficiency is due, in part, to the uncertain rates and mechanism for the reactive dissolution of NO(2)(g) (2NO(2) + H(2)O = NO(3)(-) + H(+) + HONO) in fog and aerosol droplets. Thus, state-of-the-art models parametrize the uptake of NO(2) by atmospheric aerosol from data obtained on "deactivated tunnel wall residue". Here, we report experiments in which NO(3)(-) production on the surface of microdroplets exposed to NO(2)(g) for approximately 1 ms is monitored by online thermospray mass spectrometry. NO(2) does not dissolve in deionized water (NO(3)(-) signals below the detection limit) but readily produces NO(3)(-) on aqueous NaX (X = Cl, Br, I) microdroplets with NO(2) uptake coefficients gamma that vary nonmonotonically with electrolyte concentration and peak at gamma(max) approximately 10(-4) for [NaX] approximately 1 mM, which is >10(3) larger than that in neat water. Since I(-) is partially oxidized to I(2)(*-) in this process, anions seem to capture NO(2)(g) into X-NO(2)(*-) radical anions for further reaction at the air/water interface. By showing that gamma is strongly enhanced by electrolytes, these results resolve outstanding discrepancies between previous measurements in neat water versus NaCl-seeded clouds. They also provide a general mechanism for the heterogeneous conversion of NO(2)(g) to (NO(3)(-) + HONO) on the surface of aqueous media.

Optical absorptivity versus molecular composition of model organic aerosol matter.

Aerosol particles affect the Earth's energy balance by absorbing and scattering radiation according to their chemical composition, size, and shape. It is generally believed that their optical properties could be deduced from the molecular composition of the complex organic matter contained in these particles, a goal pursued by many groups via high-resolution mass spectrometry, although: (1) absorptivity is associated with structural chromophores rather than with molecular formulas, (2) compositional space is a small projection of structural space, and (3) mixtures of polar polyfunctional species usually exhibit supramolecular interactions. Here we report a suite of experiments showing that the photolysis of aqueous pyruvic acid (a proxy for aerosol alpha-dicarbonyls absorbing at lambda > 300 nm) generates mixtures of identifiable aliphatic polyfunctional oligomers that develop absorptions in the visible upon standing in the dark. These absorptions and their induced fluorescence emissions can be repeatedly bleached and retrieved without carbon loss or ostensible changes in the electrospray mass spectra of the corresponding mixtures and display unambiguous signatures of supramolecular effects. The nonlinear additivity of the properties of the components of these mixtures supports the notion that full structural speciation is insufficient and possibly unnecessary for understanding the optical properties of aerosol particles and their responses to changing ambient conditions.

Ozonolysis of uric acid at the air/water interface.

Uric acid (UA) epoxide, peroxide, and ozonide species produced in aqueous UA microdroplets exposed to O(3)(g) are detected by online mass spectrometry within approximately 1 ms. UA conversions are independent of its initial concentration below approximately 0.1 mM and are unaffected by addition of excess H(2)O(2) or t-butanol. UA reactivity increases approximately 380 times from pH 4 to 7, which is at variance with the pH-independent rates reported for the UA + O(3)(aq) reaction in bulk water. At pH approximately 7, UA and ascorbic acid (AH(2)) microdroplets react with O(3)(g) at similar rates, although UA is approximately 40 times more reactive than AH(2) toward O(3)(aq) in bulk water. Only the UA epoxide, plus traces of UA peroxide, are formed upon mixing UA(aq) and O(3)(aq) solutions. We infer that the gas-liquid ozonolysis of UA proceeds in an interfacial aqueous medium quite distinct from bulk water. Thus, UA, a component of the pulmonary epithelial lining fluid that scavenges atmospheric O(3)(g) into less deleterious species (similar to AH(2)), is rendered inactive below pH approximately 5. The potential implications of these findings on synergistic health effects between tropospheric ozone and acidic particulates are briefly analyzed.

Anion fractionation and reactivity at air/water:methanol interfaces. Implications for the origin of hofmeister effects.

Anions are selectively enriched in interfacial layers. This universal phenomenon, first identified in connection with protein precipitation 120 years ago, underlies fundamental processes. Its physical causes, however, remain conjectural. It has been speculated that the more polarizable anions should have larger affinities for air/liquid interfaces, and that their reactivities toward gaseous species would be affected by whether the liquid is capped by hydroxyl groups, as in water itself, or by hydrophobic layers of organic contaminants. These issues are particularly relevant to the composition and fate of atmospheric aerosols. Recently, we found that fractionation factors, f X (-) , of simple anions at the air/water interface increase exponentially with ion radius, a X (-) . In this paper, we report new experimental results on a set of anions that include the large PF 6 (-) and the highly polarizable IO 3 (-) species. A strict ln f X (-) proportional, variant a X (-) correlation is confirmed. Experiments performed in { x wH 2O + (1 - x w)MeOH} mixtures show that f X (-) is almost independent of x w. Furthermore, O 3(g) oxidizes I (-) at virtually identical rates on H 2O and MeOH.